Binaural interactions in primary auditory cortex of the awake macaque

The functional organization of primary auditory cortex in non primates is g enerally modeled as a tonotopic gradient with an orthogonal representation of independently mapped binaural interaction columns along the isofrequency contours. Little information is available regarding the validity of this m odel in the primate brain, despite the importance of binaural cues for soun d localization and auditory scene analysis. Binaural and monaural responses of A1 to pure tone stimulation were studied using auditory evoked potentia ls, current source density and multiunit activity. Key findings include: (i ) differential distribution of binaural responses with respect to best freq uency, such that 74% of the sites exhibiting binaural summation had best fr equencies below 2000 Hz: (ii) the pattern of binaural responses was variabl e with respect to cortical depth, with binaural summation often observed in the supragranular laminae of sites showing binaural suppression in thalamo recipient laminae; and (iii) dissociation of binaural responses between the initial and sustained action potential firing of neuronal ensembles in A1. These data support earlier findings regarding the temporal and spatial com plexity of responses in A1 in the awake state, and are inconsistent with a simple orthogonal arrangement of binaural interaction columns and best freq uency in A1 of the awake primate.